Materials that respond sensitively to temperature variations are used in several applications that range from electrical temperature sensors to micro bolometers for thermal cameras.
Existing high-performance temperature-sensitive materials, such as vanadium oxide, have temperature coefficients of electrical resistance (TCR) on the order of −6% K−1 at room temperature. These materials derive their properties from changes of their crystal structure during semiconductor to metal transitions.
Variations of the ambient temperature influence the biopotential of living plants. Experiments performed in vivo on a maple tree (Acer saccharum) showed an exponential correlation between the tree's branch electrical resistance and temperature. This behaviour has been attributed to ionic conductivity occurring in plant cell walls.
The plant cell wall, positioned outside the plasma membrane, is composed of carbohydrates such as cellulose microfibrils with diameters as small as 3.0 nm, and hemicellulose interconnected with pectin. Pectins are composed of pectic polysaccharides rich in galacturonic acid that influence properties such as porosity, surface charge, pH, and ion balance and therefore are critical for ion transport within the cell wall. Pectins contain multiple negatively charged saccharides that bind cations, such as Ca2+, that form cross-links that confer strength and expansibility to the cell wall. It has been shown that the gelation rate of pectin decreases exponentially with temperature so that the number of dissociated chains is higher at elevated temperatures. This finding is explained by an entropic effect: as the temperature increases, the probability of interaction between two pectin chains is reduced (Cardoso et al. (2003), Food Hydrocoll 17(6):801-807).
Here, we utilized the exceptionally high temperature- and moisture sensitivity of biopolymers and related materials to develop temperature and humidity sensors with unique properties.
The objective of the present invention is to provide a temperature sensor comprising a polymer gel.
This objective is attained by the subject-matter of the independent claims.